In an MOS (metal oxide semiconductor) type thin film transistor (referred to as MOS-TFT hereinafter), the effective mobility .mu. of a carrier can be increased if an active layer made of a polysilicon film is adopted, compared with the case of adopting an active layer made of an amorphous silicon film. Also, the active layer made of a polysilicon film allows the use of a high-temperature heating process in manufacturing the MOS-TFT.
However, in the case of adopting the active layer made of a polysilicon film, because a large number of traps exist in the polysilicon film, there arises such drawbacks that the threshold voltage V.sub.T of the MOS-TFT and a gate voltage for operating the MOS-TFT increase.
As a countermeasure, in order to reduce the trap density of the polysilicon film, the following method is adopted.
First, MOS-TFT is formed. Secondly, a plasma silicon nitride film containing hydrogen is formed by the plasma chemical vapor deposition (CVD) method. Finally, the plasma silicon nitride film is annealed (see Japanese Examined Patent Publication No. 57098/1992 (Tokukohei 4-57098)). FIG. 3(a) and FIG. 3(b) illustrate an example of a structure of a semiconductor device disclosed in the above publication. Here, the semiconductor device is employed in an active-matrix type liquid crystal display device. FIG. 3(a) is a plan view showing a thin film transistor (TFT) portion constituting a matrix circuit section of the liquid crystal display device, and FIG. 3(b) is a cross sectional view showing the structure of the thin film transistor (TFT) portion taken along a line C-C' in FIG. 3(a) As shown in FIG. 3(a) and FIG. 3(b), the matrix circuit section of the liquid crystal display device is composed of the TFT 300. On a glass substrate 301 constituting the liquid crystal display device, a light-shielding film 302 is provided so as to correspond to each TFT 300. An insulating film 303 is provided on each light-shielding film 302 so as to cover the light-shielding film 302. A portion on the insulating film 303 where TFT 300 is provided is provided with a semiconductor layer 304 constituting the TFT. The semiconductor layer 304 is composed of a channel region 304a, a source region 304b.sub.1, and a drain region 304b.sub.2.
On the semiconductor layer 304, a gate insulating film 305 made of SiO.sub.2 or other materials is provided so as to cover the semiconductor layer 304. On the gate insulating film 305, a gate electrode 306 is provided so as to face the channel region 304a. The surface of the gate electrode 306 is covered with an anodic oxidation film 307. Further, on the gate electrode 306, an interlayer insulating film 308 is provided so as to cover the entire surface of the gate electrode 306. On portions of the interlayer insulating film 308 corresponding to the source region 304b.sub.1 and the drain region 304b.sub.2, a source electrode 309 and a drain electrode 310 are provided respectively. The source electrode 309 and the drain electrode 310 are electrically connected respectively to the source region b.sub.1 and the drain region b.sub.2 via contact halls 313. Further, the drain electrode 310 is electrically connected to a transparent pixel electrode 311 provided on the interlayer insulating film 308.
The entire surface of the TFT 300 formed in the described manner is covered with a SiN.sub.X film (passivation film) 312 which has been formed by a plasma CVD device. The SiN.sub.X film 312 is obtained by the steps: providing a plasma silicon nitride film containing hydrogen; and annealing the plasma silicon nitride film. The hydrogen atoms contained in the plasma silicon nitride film move into the polysilicon film and adhere to the traps that exist in the polysilicon film, thereby reducing the trap density of the polysilicon film forming the semiconductor layer 104 which constitutes the TFT 300.
However, because the SiN.sub.X film 312 formed by the plasma CVD method has low throughput and low productivity, the cost of manufacturing the SiN.sub.X film 312 becomes high if the SiN.sub.X film 312 is to be provided on the entire surface of the TFT 300. In other words, in order to reduce the manufacturing cost, it is required to cut down the number of manufacturing steps. Hence, it is difficult to reduce the manufacturing cost if the above-noted SiN.sub.X film 312 is to be provided.